Hot-rolled steel sheet and method for producing same

Abstract

A hot rolled steel sheet has a chemical composition including, by mass %, C: 0.060% to 0.120%; Si: 0.10% to 0.70%; Mn: 1.00% to 1.80%; P: 0.10% or less; S: 0.010% or less; Al: 0.01% to 0.10%; N: 0.010% or less; Nb: 0.010% to 0.100%, wherein Nb is contained so that content of solute Nb is 5% or more relative to the total Nb content; the balance being Fe and incidental impurities. The hot rolled steel sheet has a microstructure containing ferrite of not more than 15 μm in average crystal grain diameter by a volume fraction of not less than 75%, the balance being low-temperature-induced phases. The hot rolled steel sheet can be suitably utilized for manufacturing a cold rolled steel sheet or hot-dip galvanized steel sheet having a tensile strength of 590 MPa or more, excellent in material homogeneity and capable of giving excellent cold rolling property.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This is the U.S. National Phase application of PCT / JP2012 / 006191, filed Sep. 27, 2012, and claims priority to Japanese Patent Application No. 2011-215615, filed Sep. 29, 2011, the disclosures of each of these applications being incorporated herein by reference in their entireties for all purposes.FIELD OF THE INVENTION[0002]The present invention relates to a hot rolled steel sheet for manufacturing a cold rolled steel sheet or hot-dip galvanized steel sheet and, in particular, to a hot rolled steel sheet with high yield ratio and excellent material homogeneity and capable of giving excellent cold rolling property, which is suitable, for instance, for members as used in the field of automotive industry. The present invention also relates to a method for producing / manufacturing the same. Here, the term yield ratio (YR) refers to an index representing the ratio of yield strength (YS) relative to tensile strength (TS), which can be expressed ...

AI Technical Summary

Benefits of technology

[0010]Therefore, the present invention aims to solve the problems of the prior art as noted above and provide a hot rolled steel sheet which can be suitably utilized for manufacturing a cold rolled steel sheet or hot-dip galvanized steel sheet having a tensile strength of 590 MPa or more, excellent in material homogeneity and capable of giving excellent cold rolling property.

[0011]The inventors conducted extensive study for obtaining a hot rolled steel sheet suitable as members for manufacturing a cold rolled steel sheet or hot-dip galvanized steel sheet that is excellent in material homogeneity and capable of giving excellent cold rolling property and has a high yield ratio, and successfully arrived at a novel recognition as follows. Namely, after completion of hot rolling, when the hot rolled steel sheet is cooled to room temperature condition, if at least 5% of Nb is contained in the steel sheet in a solute condition without being fully precipitated as carbonitride, and the solute Nb in the hot rolled steel sheet is precipitated as carbonitride in successive annealing process, the precipitation of Nb occurs minutely in the steel sheet to suppress fluctuation in strength and elongation properties of the material. It has also been found that, by controlling the average crystal grain diameter of ferrite in a hot rolled steel sheet, it is possible to secure strength after annealing and suppress fluctuation of the material properties.

[0012]It has been further found that if the solute Nb is contained by 5% or more in the hot rolled steel sheet and the volume fraction of ferrite is controlled to not less than 75%, an advantageous effect is achieved as follows. That is, it is then possible to prevent an excessive strengthening as a result of precipitation-strengthening, or due to hard phase in the form of low-temperature-induced phases, thereby improving the cold rolling property. Based on such findings, it is possible to create a hot rolled steel sheet which can be suitably utilized as material for manufacturing a cold rolled steel sheet and hot-dip galvanized steel sheet having high yield ratio with stable strength and elongation properties after annealing.

[0020]According to the present invention, it is possible to provide a hot rolled steel sheet which can be suitably utilized as material for manufacturing a cold rolled steel sheet and hot-dip galvanized steel sheet excellent in material homogeneity and capable of giving excellent cold rolling property and having a high yield ratio. The cold rolled steel sheet and hot-dip galvanized steel sheet obtained by utilizing the hot rolled steel sheet according to the present invention is suitably applicable, for instance, to vehicle structural members, allowing improvement in fuel efficiency due to reduced weight of the vehicle body while ensuring satisfactory collision safety characteristics required for automobiles. Furthermore, it is also possible to reduce the rolling load during cold rolling, leading to improved mass-production stability.

Problems solved by technology

Although Patent Literatures 1 and 2 disclose a method for manufacturing a hot rolled steel sheet having high ductility or hole expansion formability, they fail to pay any attention to a hot rolled material for manufacturing a cold rolled steel sheet focused, in particular, to a cold rolling property, or for manufacturing a hot-dip galvanized steel sheet.